This invention relates in general to the art of pickling aluminum articles and more particular to a new process for pickling the same.
Articles of aluminum are often pickled at their surfaces in order to remove undesired oxide layers. This pickling operation may serve to obtain a more attractive appearance but also to give the pickled surface better characteristics for making adhesive joints or welded joints or to prepare it for an anodizing treatment.
The pickling operation is usually carried out by immersion of the article into an acid bath which comprises an oxidation agent in addition to a strong acid. Baths of chromic acid and sulfuric acid, as well as baths of sodium bichromate and sulfuric acid have been found to be most suitable for this purpose. However, although excellent results may be achieved in this way, the use of such pickling baths has become more and more objectionable in these days in view of the fact that the exhausted baths, due to their chromic acid or bichromate content, are most harmful to the environment and may, therefore, not be discharged to a sewer or to surface water in a direct way.
During a search for alternative pickling methods, it has been found that the same good results as in the past may be achieved with a pickling bath of sulfuric acid alone, provided that a slight anodic control voltage be applied to the bath. However, such anodic control voltage will complicate the method and will always cost energy. Therefore, it would be preferable if a method could be found wherein the control voltage is not needed.
The invention has for its object to provide a pickling process for aluminum articles wherein the use of chromic acid or bichromate in the pickling bath may be eliminated without any need for an anodic control voltage.
A further object is to provide a pickling process for aluminum articles wherein pickled articles of excellent quality can be obtained without the use of chromic acid or bichromate in the pickling bath and without any anodic control voltage.
In accordance with the present invention, these objects are realized by providing a pickling bath comprising carbon particles suspended in sulfuric acid without any chromic acid or bichromate being present. The carbon particles are preferably kept in suspension by agitating the bath continuously. Then, an aluminum article to be pickled is immersed into the bath and is maintained therein for a period of time sufficient to reach complete pickling of the article. As soon as the aluminium article has contacted one of the suspended carbon particles of the bath, a tiny electric cell having the aluminium article as an anode and the carbon particle as a cathode is formed. This cell will initiate an electrochemical dissolution phenomena at the surface of the aluminum article. Since a plurality of suspended carbon particles is present in the bath, many of these particles will contact the aluminum article and will form a corresponding number of tiny cells which will initiate the dissolution phenomena at the surface of the article. As a result thereof, the electrochemical dissolution phenomena is occurring at generally the entire immersed surface of the aluminum article and chemical attack the sulfuric acid of the bath is intensified, thereby, in an efficient way. When the aluminum article is maintained in the bath for a sufficient period of time, a complete pickling of the surface of the article may be achieved, in spite of the fact that the bath does not contain chromic acid or bichromate and in spite of the absence of an anodic control voltage.
In a copending patent application of even date entitled "Pickling Of Aluminum", there has been suggested positioning an aluminum article as an anode in a bath of sulfuric acid without chromic acid or bichromate, and then connecting this article by an external short-circuit connection to a carbon cathode placed in the same bath. In this case, proper pickling can be achieved as well, since a current will flow through the short-circuit connection and will initiate the electrochemical dissolution phenomena at the surface of the aluminum anode. Thus, chemical pickling by sulfuric acid is intensified by the electrochemical phenomena. The present invention differs from the copending case by the fact that a suspension of carbon particles instead of a carbon body is used and by the fact that the external short-circuit connection is not needed here.
Further characteristics of the invention process will be apparent from the following part of the specification.
According to the present invention, it is possible to obtain substantially equal results as with baths of chromic acid-sulfuric acid or bichromate-sulfuric acid. An important advantage is that chromic acid or bichromate are no longer required and this means that problems inherent to the disposal of exhausted baths are notably reduced. Another advantage is that an anodic control voltage is no longer required and this means that the apparatus may remain rather simple and that no energy supply is needed.